1. Field of the Invention
This invention relates to a device for controlling variable pitch vanes in a turbomachine, such as an aircraft turbojet or turboprop engine.
2. Description of the Related Art
A turbomachine includes one or several stages of variable pitch straightening vanes which are mounted between rotor wheels of a compressor. These variable pitch vanes are borne by the stator of the turbomachine and can be adjusted in position around their axes in order to optimize the gas flow in the engine of the turbomachine.
A variable pitch vane includes a cylindrical pivot at each of its ends, these pivots defining the axis of rotation of the vane. The radially external pivot is engaged in a cylindrical duct of a casing of the turbomachine and is connected by a link to a control ring driven in rotation around the casing by a jack. The rotation of the ring is transmitted by the links to the external pivots of the vanes and makes them rotate on their axes.
The piston rod of the jack can be connected directly to an articulation point on the ring. When the cylinder of the jack is mounted fixed on the casing of the turbomachine, the axis of the jack must be as tangential as possible to the ring, in order to limit the transverse forces applied on the rod of the jack during the displacement of the ring. The maximum admissible misalignment is +/−6° between the axis of the piston rod of the jack and a tangent to the ring, on the aforementioned articulation point, so that the transverse forces applied on the rod of the jack show less than about 10% of the axial force transmitted by the jack. It is therefore sometimes impossible to raise the jack in the immediate vicinity of the articulation point on the control ring, and therefore the rod of the jack must be connected to the control ring via a connecting rod. However, in this case, the misalignment of the axes of the jack and of the connecting rod can reach about 30°, and even more, which generates transverse forces that are too high on the rod of the jack during its displacements.